Wasserschloss (engineering)

Niederwartha pumped storage plant : downpipes with the water locks

Saalach power station Bad Reichenhall, moated castle; built around 1910

A moated castle is a hydraulic engineering system in the headrace of hydropower plants and other water pipes that overcome greater gradients. It is used to reduce the water hammer in pressure tunnels and pressure lines that occurs when valves are closed or opened in the line.

The name goes back to antiquity: The Romans built large well houses ( nymphaea ) at the end of their large water pipes ( aqueducts ). According to Vitruvius' description , a distribution structure was usually connected upstream, which also included an elevated storage tank. As a side effect, this reduced pressure surges . These buildings were - like the spring system  - often designed particularly splendidly and were therefore called Castelli (Latin / Italian, castles ).

general description

The moated castle is a watertight structure or (cylindrical) container which - in contrast to an  air chamber - is connected to the surrounding atmosphere through ventilation openings. The inlet of the surge tank is branched off from the actual headrace route and enters the tank from below. The water level in the water lock corresponds in the balanced state of rest on the principle of communicating tubes with the water level at the engine water inlet of the upper reservoir . The headwater does not flow directly through the moat, but inside the water level the water level can level out freely for pressure equalization. The Niederwartha pumped storage plant in Dresden, for example, uses three cylindrical water tanks (see photo) because three pressure pipes are operated independently there.

A surge tank is necessary to compensate and dampen pressure fluctuations in the pressure pipes of a hydropower plant ( storage power plant or pumped storage power plant ) . In a longer pipeline there is a large amount of water with a correspondingly high kinetic energy . If the pipeline is closed with a valve , this amount of water is slowed down. A very high pressure occurs briefly on the slide, while a negative pressure is created behind the slide . These pressure surges can damage valves, pipes and their components.

A water lock dampens the pressure surges by diverting the flowing water. As a result, the braking process is lengthened and moderated. When the slide is closed, the water gives way to the water lock, and the water level oscillates up and down until it comes to rest. The kinetic energy of the water flowing in the pipe is converted into potential energy when the water rises in the water lock.

A moated castle is built on hydropower plants at the upper end of the drop shaft. It has to stand high so that the water does not leak out of the upper opening. The upper edge must be higher than the highest water level of the upper basin and the energy line of the headrace.

Load on the hydropower plant (Schukowski shock)

The complete or partial closure of pipelines ( e.g. to regulate turbines ) causes pressure surges in pressure lines due to the rapid change in volume flow .

As pressure surges a multiple of the operating pressure can reach, it must load especially in the design of high-pressure water turbines are considered. This can be done by increasing the cross-section of the feed line (reducing the flow rate). Pressure surges for the pipeline can be rendered harmless particularly effectively by means of an atmospherically open storage volume. In the case of systems with long headrace tunnels, the surge tank is provided at the end of the adit, if possible.

The speed of pressure propagation in water ${\displaystyle a}$

${\displaystyle a={\dfrac {1}{\sqrt {{\dfrac {\rho }{K_{\mathrm {w} }}}+{\dfrac {\rho }{E_{\mathrm {R} }}}\cdot {\dfrac {d}{s}}}}}}$

is around 1000 m / s, depending on

• the density of water with , ${\displaystyle \rho }$ ${\displaystyle \rho \approx 1000\,\mathrm {\tfrac {kg}{m^{3}}} }$
• the modulus of elasticity of the pipeline, ${\displaystyle E_{\mathrm {R} }}$
• their inner diameter , ${\displaystyle d}$
• their wall thickness , ${\displaystyle s}$
• the compression module of water with . ${\displaystyle K_{\mathrm {w} }=2{,}08\cdot 10^{9}\,\mathrm {\tfrac {N}{m^{2}}} }$

The size of the pressure surge depends on the change in speed per unit of time. In the event of an emergency shutdown (speed difference between flow speed and zero), the maximum pressure surge can develop, which is also called Schukowski shock after Nikolai Jegorowitsch Schukowski : ${\displaystyle =\mathrm {\Delta } u_{\mathrm {max} }}$ ${\displaystyle u_{\mathrm {Betrieb} }}$ ${\displaystyle \mathrm {\Delta } p_{\mathrm {max} }}$

${\displaystyle \mathrm {\Delta } p_{\mathrm {max} }=a\cdot \mathrm {\Delta } u_{\mathrm {max} }\cdot \rho }$

This maximum pressure surge occurs, however, only if the closing time of the shut-off device is shorter than the time required for the pressure to get to the inlet of the headrace pipe and back to the closure (i.e. twice the pipe length ) - the reflection time of the pressure wave : ${\displaystyle 2\cdot L}$ ${\displaystyle t_{\mathrm {R} }}$

${\displaystyle t_{\mathrm {R} }={\dfrac {2\cdot L}{a}}}$

If the closing time increases, the pressure surge is lower because the flow is partially throttled within the reflection time. ${\displaystyle t_{\mathrm {R} }}$

As can be seen by inserting typical values, pressure surges usually have a magnitude of several bar that must be intercepted or reduced.

Instead of rapidly reducing the speed in the event of a quick shutdown of the shut-off devices, the water can escape into the chamber of the surge tank. Kinetic energy in the pressure tunnel is converted into potential energy in the surge chamber. The resulting counter pressure brakes the water in the pressure tunnel and causes it to flow back in the opposite direction. The water flowing back and forth creates an oscillating movement of the water level in the chamber, which gradually dampens. This surge tank oscillation represents a sluggish mass oscillation compared to the pressure surge. The maximum pressure occurring in the pressure tunnel now corresponds to the maximum water level in the surge tank. The section of the entire supply line affected by the pressure surge then extends only to the area of ​​the pressure shaft. The amplitude of the shock wave is usually below the maximum Schukowski shock, even with a short expansion of the shaft, due to the relatively long closing times at the time of the shock wave.

Types

After the structural training

The four moated towers of the Mosul dam

Moated castle buildings

This type of water lock is used for penstock pipelines that are close to the surface or at ground level. The tower-like structures are made of steel or reinforced concrete.

Manhole, tunnel or cavern water locks

In the case of systems with headrace tunnels or pressure shafts running underground, the surge tank is placed in caverns , in particular in the form of a shaft.

Mixed construction

As a result of the low coverage of the pressure line, a combination of an underground shaft structure and a high-rise water tower may be necessary.

According to the hydraulic mode of operation

Shaft lock

The shaft lock is the simplest form in which the swing between the pipeline and the chamber is possible without hindrance.

Aqueous lock

In contrast to the shaft water lock, the chamber water lock can be designed with smaller structural dimensions in control mode. The more complex construction has a disadvantage.

Throttle water lock

In order to reduce the amplitude of the rising water level and thus to be able to dimension the chamber smaller, the cross-section at the base of the surge tank is narrowed in order to create a flow resistance . The counterpressure builds up faster and the damping increases.

Differential water lock

The differential water lock behaves similarly to the throttle water lock, but here a rapid pressure increase is achieved through the separately arranged riser shaft.

Surge chamber

The surge chamber is a facility similar to the moated castle. This pressure equalization structure, which is arranged in the underwater (below a hydropower plant), is required if the pipeline opens into a body of water with a low water level.

Another application arises in long gravity tunnels in the underwater, which sometimes act as pressure tunnels due to strong load fluctuations.

See also

• Hydraulic ram

literature

• Charles Jaeger: Technical hydraulics . Verlag Birkhäuser, Basel 1949, pp. 175 ff. (Section 7).

Web links

• Deutsches Museum: see pumped storage operation
• Example from an Italian dam side (drawing; Pozzo piezometrico = moated castle)
• Flooding of an upper surge tank after the three turbines have been switched off
• Inspection of the same moated castle

Individual evidence

1. ↑ Klaus Kramer: Installer - a craft with a history: a picture sheet of the sanitary culture from the origins to the modern age . Ed .: Hans Grohe GmbH (=  Hans Grohe series of publications . Volume 2 ). Klaus Kramer Verlag, Schiltach 1998, ISBN 3-9805874-2-8 , p. 43–44 ( limited preview in Google Book search).
2. ^ ^{A b} T. Strobl, F. Zunic: Hydraulic engineering, current bases - new developments. Springer-Verlag, Berlin / Heidelberg / New York 2006, ISBN 3-540-22300-2 .
3. ↑ ^{a b} J. Giesecke, E. Mosonyi: Hydropower plants, planning, construction and operation. Springer-Verlag, Berlin / Heidelberg / New York 2005, ISBN 3-540-25505-2 .